The current method of designing and constructing primary airliner structures, such as fuselages, is called semi-monocoque. The semi-monocoque structure contains thousands of detail parts riveted into substantially transverse stiffeners or ribs (also called frames) and into substantially longitudinal stiffeners or ribs (also called stringers), both of which are riveted to the skin. Although semi-monocoque structures have been successful for their intended purpose, it would be even more desirable to provide structures that are even less labor intensive, costly, and time-consuming to design, fabricate and assemble.
More specifically, the design phase for a traditional semi-monocoque jetliner requires designing thousands of details, splices, and assemblies and specifying a whole range of rivet types, their respective locations and spacing. Fabricating thousands of detail parts and maintaining configuration control while doing so can be extremely complicated. Accordingly, a great number of fabrication shops are typically employed to fabricate the various parts of a semi-monocoque design for a major airliner. Indeed, it is not uncommon for a commercial jetliner to contain about three million (3,000,000) holes drilled through parts with an equal number of fasteners installed, which is all done and orchestrated through and by over one thousand (1000) fabrication shops.
Assembling a conventional semi-monocoque aircraft involves riveting thousands of detail parts into the frames and stringers, which in turn are riveted into the axial load carrying skins. The complex assembly of thousands of such detail parts requires specifications, fabrication, and tracking of a whole range of rivet types and small parts, their locations, spacings, etc. for fastening the thousands of parts into the semi-monocoque structure. Accordingly, a great number of assembly shops are typically needed to assemble a conventional semi-monocoque aircraft. Indeed, it is not uncommon for nearly one hundred (100) assembly shops to be involved in the assembly of a commercial jetliner.
In view of the foregoing, it will be readily apparent that it would be highly beneficial to provide an aircraft structure that may be assembled with significantly less fastener holes, while those fastener holes which are still employed would facilitate a fully determinately assembled product requiring no or little drilling at the assembly operation.
Historical studies of aircraft indicate that fastener holes are the source or origination location for nearly all fuselage cracks, which tend to reduce the service life of the airframe. In addition, it is also known that fastener holes are the major culprit in the development of multi-site fatigue damage, fretting corrosion, and costly aircraft inspection, refurbishing and maintenance. With less fastener holes, less time would be needed for conducting routine inspections of and for repairing fastener holes to ensure the structural integrity of the airliner.
Providing an aircraft structure that is even less costly to design, fabricate, and assemble than the current semi-monocoque structures would be financially beneficial to both airframe manufacturers from the fabrication standpoint and to airline operators from the “Cost of Ownership” and maintenance standpoints. It is well known in the industry that “Cost of Ownership” has become the largest single fixed component of operating jetliner aircraft. The “Cost of Ownership” burden is shifting rapidly to the aircraft manufactures with the increasing airline industry trend toward leasing rather than owning jetliners.
Although airline structure costs and the affordability thereof are dependent at least in part on the time and labor required for and complexities associated with the design, fabrication, and assembly of the aircraft structure, other factors also are important. Aircraft structure costs, affordability and to a degree weight are driven not only by large part counts, but also by failsafe considerations and by stringer splice repair procedures. On the one hand, there is the increased cost of designing, testing and life demonstration; and on the other hand, wherever service life may be limited or reduced, there is naturally the increased cost of inspections and stringer splice repairs associated with large numbers of fastener holes.
Furthermore, the number of aircraft manufacturers owning commercial aircraft has increased as a result of the increasing trend of airline operators to lease rather than own commercial aircraft. Accordingly, it would be beneficial to such aircraft manufacturers to increase the service life and economically viable life limit of operating their commercial jetliner inventory. Thus, it would be beneficial to provide structures that are even more durable and damage-tolerant and have increased fatigue capabilities.
Accordingly, a need remains for methods of making skin panels that are suitable for fuselages and other primary aircraft structure wherein the skin panels and the aircraft structures formed therewith are even less labor-intensive and costly to design, fabricate, assemble, inspect and repair than existing designs. Ideally, the methods would provide skin panels that are capable of being used to form more durable and damage-tolerant structures with increased fatigue capabilities.
Regarding fuel tank structures, conventional fuel tank structures typically do not have as many parts as conventional semi-monocoque fuselages. However, the methods currently used for making fuel tank skin panels can be costly especially when the skin panels must be provided with biaxial curvatures. For example, a biaxially curved skin panel is needed for propellant tank dome or rounded end portion. Although machining of thick plates and spin forming are successful at providing biaxial curvatures to fuel tank skin panels, it would be beneficial to provide a less costly method of making the same. Accordingly, a need remains for a less costly and more efficient process for making biaxially curved skin panels that are suitable for use with fuel tank structures such as aircraft fuel tanks and RLV propellant tanks.